In recent years the pollution of air, water and/or soil has become a key issue, especially in urban areas. Two of the major classes of environmental polluting substances are nitrogen oxides (NOx) and volatile organic compounds (VOCs). These compounds are, in particular, considered as undesirable because they initiate the formation of secondary polluting substances.
Various solutions have been proposed to reduce the concentration of air polluting substances in the environment.
The photocatalytic properties of titanium dioxide (TiO2) are well known. Titanium dioxide activates with ultraviolet (UV) radiation to oxidize air pollutants, such as nitrogen oxides and volatile organic compounds. Therefore titanium dioxide has been employed to remove harmful NOx gases (NO and NO2) from the atmosphere, by oxidation to nitrates.
For example, applying titania particles to pavement can help to remove emission pollutants right next to the source, i.e., near the vehicles. The degradation of nitrogen oxides by concrete paving stones that contain titania has been studied and is described in M. M. Ballari et al. Applied Catalysis B: Environmental, 95, (2010), 245-254.
Photocatalytic surfaces containing titania particles, e.g., paints or plastic coatings, promote NOx removal by photodestructive radical mechanisms. Therefore, related photodestruction of other materials in the coating, especially polymers, has been an unavoidable consequence. These photocatalytic properties of titanium dioxide particles therefore result in very short lifetimes for coatings containing the titania.
To date, it has been difficult to provide coatings having high levels of titanium dioxide photocatalyst because the catalyst tends to oxidize and break down polymeric material (e.g., binder) in the coating. This problem is exacerbated when the coating is exposed to intense UV radiation from sunlight, as is the case with an exterior paint.
For exterior paints, their activity in terms of ability to remove pollutant gases may be reduced after weathering.
Equally, for exterior paints, significant weight loss will normally occur after weathering, due to the titania breaking down the polymeric binder of the paint. Gloss loss and color fade (in non-white surfaces) are more evident symptoms of degradation.
Thus it has been identified by the inventors that there is a need for coating compositions which can exhibit excellent surface de-polluting properties without the coating undergoing significant photodegradation.
It would be desirable for these characteristics to be present regardless of the environment to which the coatings are exposed. Thus they should be present both for coatings that are used indoors and for coatings that are exposed to intense UV radiation from direct sunlight.